build: remove explicit -shared in LDFLAGS

This doesn't need to be passed, as it's handled by libtool. Since the
default for autoconf is --disable-static, this change is effectively a
noop. It only matters if you pass --enable-static, in which case the
static libs for systemd libraries will actually be built.

Nitpicky, but this only affects systemd libs. The override for the
other libs remains since these libs are always loaded dynamically and
never compiled staticly.