element: remove inactive pad g_warning in add_pad

element: remove inactive pad g_warning in add_pad

The documentation incorrectly used to state that the pads were
not automatically activated when added, whereas we actually do
that when appropriate.

Callers of gst_element_add_pad must not hold the object lock,
which implies that they cannot perform the same checks as
add_pad in a non-racy manner.

This updates the documentation, and removes the g_warning
that was output before performing automatic activation.

https://bugzilla.gnome.org/show_bug.cgi?id=797181