dlopen.3: Amend error in description of dlclose() behavior

dlopen.3: Amend error in description of dlclose() behavior

The current text says that unloading depends on whether
the reference count falls to zero *and no other libraries
are using symbols in this library*. That latter text has
been there since man-pages-1.29, but it seems rather dubious.
How could the implementation know whether other libraries
are still using symbols in this library? Furthermore, no
other implementation's man page mentions this point.
Seems best to drop this point.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>