Remove unsupported AC_FUNC_FSEEKO macro

[gfphoenix78]

AC_FUNC_FSEEKO macro doesn't use ac_cv_func_fseeko in configure.ac and we have to force HAVE_FSEEKO to 1. The pg upstream has cleared this part of script, but cbdb doesn't.

Reference:
postgres/postgres@799d224 https://www.postgresql.org/message-id/200802190116.m1J1GcZ28086%40momjian.us

fix #516

Why are the changes needed?

configure will report a warning/error. This commit fixes the issue and aligns the code with pg upstream.

Does this PR introduce any user-facing change?

No

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