Skin: Remove redundant use of QuickTemplate::setRef()

Skin: Remove redundant use of QuickTemplate::setRef()

As part of long-term moving towards having skins consume prepared
data and not making additional run-time database queries or additional
PHP-specific semantics and global state, try to phase out use
of QuickTemplate::setRef(), which is incompatible with the idea of
data only flowing in one direction (given it allows the value to
change at any time).

* jsmimetype: Deprecated/unused, has a constant value for compat.
* mimetype: Obsolete, Content-Type header comes from OutputPage.
* skin: Objects are references by default, '$this' can't be
  modified anyway.

Bug: T140664
Change-Id: Ia505cc3e1b774a380fb36b4f0e2522550aef5bbd