Delete unnecessary loading of 'launch.frontend.interpolate_substitution_method' entry point that was never used

[ivanpauno]

I think the original idea was to have a way of overloading the default substitution interpolation method (i.e. use sth different than $() notation), but custom parsers can already override parse_substitution method:

launch/launch/launch/frontend/parser.py

Line 92 in db2a421

def parse_substitution(self, value: Text) -> SomeSubstitutionsType:

launch/launch/launch/frontend/parser.py

Lines 78 to 85 in db2a421

	def load_parser_implementations(cls):
	"""Load all the available frontend entities."""
	if cls.frontend_parsers is None:
	cls.frontend_parsers = {
	entry_point.name: entry_point.load()
	for entry_point in importlib_metadata.entry_points().get(
	'launch.frontend.parser', [])
	}

I think I forgot deleting the loading of this entry points of the original code, because it doesn't make sense.

[ivanpauno]

Rolling PR checker is passing.
This is just removing dead code, so I will proceed to merge without running multi-platform CI.