mesa: remove _CurrentFragmentProgram from gl_pipeline_object

mesa: remove _CurrentFragmentProgram from gl_pipeline_object

This was added in b527dd65c830a as a work around because fixed function
fragment shaders were tracked in ctx->FragmentProgram._Current as
a gl_program rather than gl_shader_program.

However after my refactoring of the program and shader structs
at the end of 2016 which culminated in c505d6d85222, we no longer
need gl_shader_program to track the current program making
_CurrentFragmentProgram obsolete.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>