Remove support for using jemalloc instead of the system allocator

There was never any investigation done to determine whether using
jemalloc was actually a net benefit for nvim.  It has been a portability
limitation and adds another factor to consider when triaging issues.

src/nvim/version.c

@@ -62,12 +62,6 @@ static char *features[] = {
 "-iconv",
 #endif
 
-#ifdef HAVE_JEMALLOC
-"+jemalloc",
-#else
-"-jemalloc",
-#endif
-
 #ifdef FEAT_TUI
 "+tui",
 #else