pgsql: libpq: Revert the connection option "servicefile"

libpq: Revert the connection option "servicefile"

The following commits are reverted:
* 092f3c63efc6, libpq: Add "servicefile" connection option
* 6b1c4d326b06, psql: Add variable SERVICEFILE
* 90354030b8fc, libpq: Fix "servicefile" after fallback to system
service file

After more review, PGSERVICEFILE is sufficient for the purpose of
specifying a per-session service file. Some details around this feature
will need more attention before being actually usable, if we ever get
down to it.

One independent piece that has been committed during the v19 development
cycle is df286a5b830a, which is kept in the tree as that was an
unrelated change.

Reported-by: Sehrope Sarkuni <sehrope(at)jackdb(dot)com>
Discussion: https://postgr.es/m/CAH7T-aq+U6GDPO8mr3Gga3KzxGpAE=BSLb38x+R8fvWjzF-H8w@mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9d882c10bfb6f56a0a430872ef3040875535b06b

Modified Files
--------------
doc/src/sgml/libpq.sgml | 24 +++-------
doc/src/sgml/ref/psql-ref.sgml | 9 ----
src/bin/psql/command.c | 7 ---
src/interfaces/libpq/fe-connect.c | 66 ++-------------------------
src/interfaces/libpq/libpq-int.h | 2 -
src/interfaces/libpq/t/006_service.pl | 86 +----------------------------------
6 files changed, 13 insertions(+), 181 deletions(-)