Remove unused arenas_extend() function declaration.
Remove unused arenas_extend() function declaration.

The arenas_extend() function was renamed to arenas_init() in commit
8bb3198f72fc7587dc93527f9f19fb5be52fa553, but its function declaration
was not removed from jemalloc_internal.h.in.

8l/jemalloc,leebaok/jemalloc-4.2.1-readcode,AndroidExternal/jemalloc,AndroidExternal/jemalloc,leebaok/jemalloc-4.2.1-readcode,wqfish/jemalloc,glandium/jemalloc,leebaok/jemalloc-4.2.1-readcode,jemalloc/jemalloc-cmake,8l/jemalloc,8l/jemalloc,glandium/jemalloc,AndroidExternal/jemalloc,glandium/jemalloc,jemalloc/jemalloc-cmake,glandium/jemalloc,wqfish/jemalloc,jemalloc/jemalloc-cmake,wqfish/jemalloc,jemalloc/jemalloc-cmake,leebaok/jemalloc-4.2.1-readcode,AndroidExternal/jemalloc,wqfish/jemalloc,wqfish/jemalloc,8l/jemalloc