Remove unneeded header declarations in multixact.c
Two header declarations were related to SQL-callable functions, that
should have been cleaned up in
df9133fa6384. Some more includes can be
removed on closer inspection, so let's clean up these as well, while on
it.
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/345438.
1755524834@sss.pgh.pa.us