Remove --disable-tcache.

Simplify configuration by removing the --disable-tcache option, but
replace the testing for that configuration with
--with-malloc-conf=tcache:false.

Fix the thread.arena and thread.tcache.flush mallctls to work correctly
if tcache is disabled.

This partially resolves #580.

include/jemalloc/internal/jemalloc_preamble.h.in

@@ -111,13 +111,6 @@ static const bool config_stats =
     false
 #endif
     ;
-static const bool config_tcache =
-#ifdef JEMALLOC_TCACHE
-    true
-#else
-    false
-#endif
-    ;
 static const bool config_tls =
 #ifdef JEMALLOC_TLS
     true