mesa: remove some unused functions in the perf monitor area

mesa: remove some unused functions in the perf monitor area

Fixes the following Clang warnings.

main/performance_monitor.c:157:1: warning: unused function 'index_to_queryid' [-Wunused-function]
index_to_queryid(GLuint index)
^
main/performance_monitor.c:163:1: warning: unused function 'queryid_valid' [-Wunused-function]
queryid_valid(const struct gl_context *ctx, GLuint queryid)
^
main/performance_monitor.c:169:1: warning: unused function 'counterid_to_index' [-Wunused-function]
counterid_to_index(GLuint counterid)
^
3 warnings generated.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>