Remove unnecessary code from malloc.c. Enhance pcache1.c so that is tries

Remove unnecessary code from malloc.c.  Enhance pcache1.c so that is tries
to reuse existing pages, rather than create new pages, when SQLite is under
memory pressure.  "Memory pressure" means that SQLITE_CONFIG_PAGECACHE memory
is nearly exhausted or sqlite3_soft_heap_limit() has been reached.

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