--with-alloc={tcmalloc,jemalloc} removed from master

[Stephan Bergmann]

 From the 
<https://cgit.freedesktop.org/libreoffice/core/commit/?id=494bdd3dc32287cc01ea4e0d81438a9266425726> 
"Support for tcmalloc and jemalloc is long dead" commit message:  "So 
the only remaining effect of --with-alloc={tcmalloc,jemalloc} is to set 
FORCE_SYSALLOC in Library_sal.mk, same as --with-alloc=system does.  So 
whoever still uses the former configure switches should just switch to 
the latter."  (Notwithstanding the fact that FORCE_SYSALLOC is 
effectively dead, too.)