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.

src/test/isolation/Makefile

@@ -72,5 +72,10 @@ maintainer-clean: distclean
 installcheck: all
 	./pg_isolation_regress --inputdir=$(srcdir) --schedule=$(srcdir)/isolation_schedule
 
-check: all
-	./pg_isolation_regress --temp-install=./tmp_check --inputdir=$(srcdir) --top-builddir=$(top_builddir) --schedule=$(srcdir)/isolation_schedule
+# We can't support "make check" because isolationtester requires libpq, and
+# in fact (on typical platforms using shared libraries) requires libpq to
+# already be installed.  You could run "make install" and then run a check
+# using a temp installation, but there seems little point in that.
+check:
+	@echo "'make check' is not supported."
+	@echo "Install PostgreSQL, then 'make installcheck' instead."