mw.log.deprecate: Simplify warning message

* Remove odd MWDeprecationWarning prefix.
* Don't mention "property". All accessors targets are properties
  and saying so doesn't add much value (some of the targets are
  more thought of as constructors or methods), the name should
  be sufficient.

Change-Id: Ifc77d176fa830feccf13350b4fd9dbf8cb8d8046
mw.log.deprecate: Use mw.log.warn instead of log.warn

In qunit/testrunner.js, freshConfigCopy() temporarily clears
mw.log.warn to avoid a flood of spurious deprecation warnings.

However, as of 89a8fe4, the console was once again flooded with
deprecated notices when in debug mode because in debug mode we
load mediawiki.log.js which redefines the mw.log object and as
such the local reference here kept pointing to the original.

Change-Id: I2b560aac9c0b929c82bb628760c194c0476fe1a9
Merge "mw.log.deprecate: Simplify warning message"
Merge "mw.log.deprecate: Use mw.log.warn instead of log.warn"