ResourceLoaderStartUpModule: Remove no-op "delete isCompatible"

ResourceLoaderStartUpModule: Remove no-op "delete isCompatible"

Due to the way this function being defined as a function or var
declaration instead of a function expression assigned to a property
it can't be deleted.

JavaScript doesn't throw an error when deletion is not permitted
though, the operator returns false instead.

> delete isCompatible;
  false

We already removed 'delete startUp' from mediawiki.js in favour
of startUp = undefined; (r107402, r74325).

Change-Id: I7aa02e3f4deb3a4f00177b70978bfcb83c80988a