meta: Remove support for clearing integer buffers

meta: Remove support for clearing integer buffers

Since i965 no longer uses this function for clearing color buffers,
there is no driver left that will ever support integer textures and use
_mesa_meta_glsl_Clear.

As a side note, the has_integer_textures check was rubbish anyway
because meta always smashes the API to API_OPENGL_COMPAT.

   text     data     bss      dec    hex  filename
12244406 1344936 1290748 14880090 e30d5a  before/lib64/dri/i965_dri.so
12243574 1344936 1290748 14879258 e30a1a  after/lib64/dri/i965_dri.so

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/856>