pgsql: Fix some compiler warnings in aset.c and generation.c

Fix some compiler warnings in aset.c and generation.c

This fixes a couple of unused variable warnings that could be seen when
compiling with MEMORY_CONTEXT_CHECKING but not USE_ASSERT_CHECKING.
Defining MEMORY_CONTEXT_CHECKING without asserts is a little unusual,
however, we shouldn't be producing any warnings from such a build.

Author: Richard Guo
Discussion: https://postgr.es/m/CAMbWs4_D-vgLEh7eO47p=73u1jWO78NWf6Qfv1FndY1kG-Q-jA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b82557ecc2ebbf649142740a1c5ce8d19089f620

Modified Files
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src/backend/utils/mmgr/aset.c       | 4 +---
src/backend/utils/mmgr/generation.c | 9 ++++++---
2 files changed, 7 insertions(+), 6 deletions(-)