Don't allow to disable backend assertions via the debug_assertions GUC.

 Don't allow to disable backend assertions via the debug_assertions GUC.
 
 The existance of the assert_enabled variable (backing the
 debug_assertions GUC) reduced the amount of knowledge some static code
 checkers (like coverity and various compilers) could infer from the
 existance of the assertion. That could have been solved by optionally
 removing the assertion_enabled variable from the Assert() et al macros
 at compile time when some special macro is defined, but the resulting
 complication doesn't seem to be worth the gain from having
 debug_assertions. Recompiling is fast enough.
 
 The debug_assertions GUC is still available, but readonly, as it's
 useful when diagnosing problems. The commandline/client startup option
 -A, which previously also allowed to enable/disable assertions, has
 been removed as it doesn't serve a purpose anymore.
 
 While at it, reduce code duplication in bufmgr.c and localbuf.c
 assertions checking for spurious buffer pins. That code had to be
 reindented anyway to cope with the assert_enabled removal.