nir: Remove empty visit_call_src and visit_load_const_src functions

nir: Remove empty visit_call_src and visit_load_const_src functions

The guts were removed in dfb3abba.  It has been almost exactly a year,
so I dont think we're going to "decide we want [predication] back."

Silences several "unused parameter" warnings:

nir/nir.c: In function ‘visit_call_src’:
nir/nir.c:1052:32: warning: unused parameter ‘instr’ [-Wunused-parameter]
 visit_call_src(nir_call_instr *instr, nir_foreach_src_cb cb, void *state)
                                ^
nir/nir.c:1052:58: warning: unused parameter ‘cb’ [-Wunused-parameter]
 visit_call_src(nir_call_instr *instr, nir_foreach_src_cb cb, void *state)
                                                          ^
nir/nir.c:1052:68: warning: unused parameter ‘state’ [-Wunused-parameter]
 visit_call_src(nir_call_instr *instr, nir_foreach_src_cb cb, void *state)
                                                                    ^
nir/nir.c: In function ‘visit_load_const_src’:
nir/nir.c:1058:44: warning: unused parameter ‘instr’ [-Wunused-parameter]
 visit_load_const_src(nir_load_const_instr *instr, nir_foreach_src_cb cb,
                                            ^
nir/nir.c:1058:70: warning: unused parameter ‘cb’ [-Wunused-parameter]
 visit_load_const_src(nir_load_const_instr *instr, nir_foreach_src_cb cb,
                                                                      ^
nir/nir.c:1059:28: warning: unused parameter ‘state’ [-Wunused-parameter]
                      void *state)
                            ^

v2: Add some comments in nir_foreach_src suggested by Jason.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Cc: Connor Abbott <cwabbott0@gmail.com>