Remove redundant const keyword

[bojanserafimov]

Everything compiles fine on our linux builds, but the compiler in our osx builds emits a incompatible-function-pointer-types warning:

/Users/runner/work/neon/neon//vendor/postgres/contrib/neon/libpagestore.c:438:15: warning: incompatible function pointer types assigning to 'dbsize_hook_type' (aka 'const long (*)(unsigned int)') from 'int64 (Oid)' (aka 'long (unsigned int)') [-Wincompatible-function-pointer-types]
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                dbsize_hook = zenith_dbsize;
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                            ^ ~~~~~~~~~~~~~
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1 warning generated.
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To me it looks like the const keyword in typedef const int64 (*dbsize_hook_type) (Oid dbOid); is redundant. An int64 returned as rvalue is const anyway, it's not a pointer. So maybe the compiler on linux ignores the const and compiles this, while the compiler on osx gets confused and emits a warning. I'm not sure if it's only confused in theory, or if it's confused enough to have undefined behavior, so I'd rather correct the definition and remove the const keyword.

I checked that this patch fixes the warning on OSx.

Should we also fail CI on compiler warnings?