libpq: Remove PQservice()HEADmaster

This routine has been introduced as a shortcut to be able to retrieve a
service name from an active connection, for psql.  Per discussion, and
as it is only used by psql, let's remove it to not clutter the libpq API
more than necessary.

The logic in psql is replaced by lookups of PQconninfoOption for the
active connection, instead, updated each time the variables are synced
by psql, the prompt shortcut relying on the variable synced.

Reported-by: Noah Misch <[email protected]>
Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 18

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