pgsql: Remove compatibility declarations for InitMaterializedSRF()

Remove compatibility declarations for InitMaterializedSRF()

These routines have been renamed in a19e5ce.  There is no need to keep
the compatibility declarations on HEAD, as once an extension moves to
the new routine name when compiling with v16~ the code would work the
same way when recompiled on v15.  No backpatch to v15 for this one,
because ABI compatibility has to be maintained there.

Discussion: https://postgr.es/m/20221013194820.ciktb2sbbpw7cljm@awork3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/eddc128beac0f6e7b87e003888b0e947f7548e8a

Modified Files
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src/backend/utils/fmgr/funcapi.c | 9 ---------
src/include/funcapi.h            | 5 -----
2 files changed, 14 deletions(-)