Refactor MTex and some clean-ups

[YishiMichael]

Proposed changes

• M manimlib/mobject/svg/mtex_mobject.py: add generate_plain_tex_file config option (default False) to allow users generate another svg without additional \color commands to prevent the shape of svg from being altered by them; some refactors for efficiency improvememt; fix the bug that multi-line tex fails rendering
• M manimlib/utils/simple_functions.py: refactor choose with lru_cache and scipy's func (borrowed from community edition); delete clip_in_place func (unused)
• M manimlib/utils/color.py: delete get_shaded_rgb func (unused)
• D manimlib/utils/string.py: clean up unused functions
• M manimlib/mobject/coordinate_systems.py: remove redundancies
• M manimlib/__init__.py: remove manimlib/utils/string.py import

[YishiMichael]

I noticed that breaking up every pair of braces may cause unexpected issues, so in the current version only tex enclosed by a pair of double-braces ({{ ... }}) are split out, making sure that these splitting are intended.
Note, this version only works after #1717 is merged.
MTex currently cannot handle tex originally including \color command. Users may use tex_to_color_map parameter instead.