pgsql: Refactor to remove ForeignServerName().

Refactor to remove ForeignServerName().

Callers either have a ForeignServer object or can readily construct
one.

Discussion: https://postgr.es/m/CAExHW5vV5znEvecX=ra2-v7UBj9-M6qvdDzuB78M-TxbYD1PEA@mail.gmail.com
Suggested-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/11f8018ee6784476d8dcee3ef64b267fb16fc374

Modified Files
--------------
src/backend/catalog/pg_subscription.c   |  7 +++++--
src/backend/commands/subscriptioncmds.c | 20 +++++++++++---------
src/backend/foreign/foreign.c           | 31 ++-----------------------------
src/include/foreign/foreign.h           |  4 ++--
4 files changed, 20 insertions(+), 42 deletions(-)