Remove "make check" target in src/test/isolation/Makefile.

This doesn't work as expected because the isolationtester program requires
libpq to already be installed.  While it works when you've already installed
libpq, having to already have done "make install" defeats most of the point
of a check with a temp installation.  And there are weird corner cases if
the dynamic linker picks up an old libpq.so from system library directories.
Remove the target (or more precisely, make it print a helpful message) so
people don't expect the case to work.