Remove TimestampTemplate configtype

The check being done only catches lone % signs, and only on some versions of
Python: https://bugs.python.org/issue35066

Since we do the real formatting in sqlite anyways, just drop it, as we can't
realistically validate it.

Fixes #4693

doc/help/settings.asciidoc

@@ -1548,8 +1548,9 @@ Default: +pass:[false]+
 [[completion.timestamp_format]]
 === completion.timestamp_format
 Format of timestamps (e.g. for the history completion).
+See https://sqlite.org/lang_datefunc.html for allowed substitutions.
 
-Type: <<types,TimestampTemplate>>
+Type: <<types,String>>
 
 Default: +pass:[%Y-%m-%d]+
 
@@ -3753,9 +3754,6 @@ See the documentation for `List`.
 
 See the setting's valid values for more information on allowed values.
 |TextAlignment|Alignment of text.
-|TimestampTemplate|An strftime-like template for timestamps.
-
-See https://sqlite.org/lang_datefunc.html for reference.
 |UniqueCharString|A string which may not contain duplicate chars.
 |Url|A URL as a string.
 |UrlPattern|A match pattern for a URL.

qutebrowser/config/configdata.yml

@@ -858,10 +858,13 @@ completion.scrollbar.padding:
 
 completion.timestamp_format:
   type:
-    name: TimestampTemplate
+    name: String
     none_ok: true
   default: '%Y-%m-%d'
-  desc: Format of timestamps (e.g. for the history completion).
+  desc: >-
+    Format of timestamps (e.g. for the history completion).
+
+    See https://sqlite.org/lang_datefunc.html for allowed substitutions.
 
 completion.web_history.exclude:
   type:

qutebrowser/config/configtypes.py

@@ -1832,31 +1832,6 @@ class NewTabPosition(String):
             ('last', "At the end."))
 
 
-class TimestampTemplate(BaseType):
-
-    """An strftime-like template for timestamps.
-
-    See https://sqlite.org/lang_datefunc.html for reference.
-    """
-
-    def to_py(self, value: _StrUnset) -> _StrUnsetNone:
-        self._basic_py_validation(value, str)
-        if isinstance(value, configutils.Unset):
-            return value
-        elif not value:
-            return None
-
-        try:
-            # Dummy check to see if the template is valid
-            datetime.datetime.now().strftime(value)
-        except ValueError as error:
-            # thrown on invalid template string
-            raise configexc.ValidationError(
-                value, "Invalid format string: {}".format(error))
-
-        return value
-
-
 class Key(BaseType):
 
     """A name of a key."""

tests/unit/config/test_configtypes.py

@@ -2084,21 +2084,6 @@ class TestConfirmQuit:
             klass().to_py(val)
 
 
-class TestTimestampTemplate:
-
-    @pytest.fixture
-    def klass(self):
-        return configtypes.TimestampTemplate
-
-    @pytest.mark.parametrize('val', ['foobar', '%H:%M', 'foo %H bar %M'])
-    def test_to_py_valid(self, klass, val):
-        assert klass().to_py(val) == val
-
-    def test_to_py_invalid(self, klass):
-        with pytest.raises(configexc.ValidationError):
-            klass().to_py('%')
-
-
 class TestKey:
 
     @pytest.fixture