Remove check-exports test.

This test is inadequate in many ways:

1. Solaris linker adds some extra global symbols like _fini or _init.
2. libtool can pass wrong option to GNU ld
   (http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13201),
   thus many other symbols are exported.

The right way to control global symbols is a version script
(in terms of GNU ld) or a map file (Solaris ld).

libtool actually generates map file for Solaris ld.