🔥 Drop libmysqlclient recipe, closes kivy#2808

The recipe was introduced in kivy#587 but has multiple issues and doesn't build.

Notable problems:
- Never updated since initial commit (aside from formatting)
- Uses `master` branch instead of pinning a version
- Contains lots of commented-out code
- Relies on https://github.com/0x-ff/libmysql-android, which:
  - Is not an official MySQL repository
  - Hasn't been updated since 2013
  - Lacks English documentation (only available in Russian)

Author: AndreMiras

ci/constants.py

@@ -33,8 +33,6 @@ class TargetPython(Enum):
     'twisted',
     # genericndkbuild is incompatible with sdl2 (which is build by default when targeting sdl2 bootstrap)
     'genericndkbuild',
-    # libmysqlclient gives a linker failure (See issue #2808)
-    'libmysqlclient',
     # boost gives errors (requires numpy? syntax error in .jam?)
     'boost',
     # libtorrent gives errors (requires boost. Also, see issue #2809, to start with)