Remove slow test which tests code which no longer exists

This test case was built specifically to test the relation key cache, a
piece of code which was removed in commit
00f0384. The code currently instead
relies on the SMGR and relation caches which this code would not really
test in any sensible way.

Author: jeltz

Makefile

@@ -6,7 +6,6 @@ DATA = pg_tde--2.0--2.1.sql pg_tde--1.0--2.0.sql pg_tde--1.0.sql
 REGRESS = \
 	access_control \
 	alter_index \
-	cache_alloc \
 	change_access_method \
 	create_database \
 	default_principal_key \