pgsql: Remove PushOverrideSearchPath() and PopOverrideSearchPath().

Remove PushOverrideSearchPath() and PopOverrideSearchPath().

Since commit 681d9e4621aac0a9c71364b6f54f00f6d8c4337f, they have no in-tree
calls.  Any new calls would introduce security vulnerabilities like the one
fixed in that commit.

Alexander Lakhin, reviewed by Aleksander Alekseev.

Discussion: https://postgr.es/m/8ffb4650-52c4-6a81-38fc-8f99be981130@gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/7c5c4e1c0396b0617a6f9b659dd7375fb0bfb9dc

Modified Files
--------------
src/backend/catalog/namespace.c  | 233 +--------------------------------------
src/backend/commands/extension.c |   5 -
src/include/catalog/namespace.h  |   2 -
src/tools/pgindent/typedefs.list |   1 -
4 files changed, 6 insertions(+), 235 deletions(-)