meson: remove useless libdl test

dlopen is never used after it is sought via cc.find_library, because
plugins use gmodule instead; remove the test.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211110092454.30916-1-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211115142915.3797652-5-alex.bennee@linaro.org>