Remove unused method CSSStyleDeclaration::makeMutable().
<http://webkit.org/b/114567>

Reviewed by Anders Carlsson.

* css/CSSComputedStyleDeclaration.cpp:
* css/CSSComputedStyleDeclaration.h:
(CSSComputedStyleDeclaration):
* css/CSSStyleDeclaration.h:
(CSSStyleDeclaration):
* css/PropertySetCSSStyleDeclaration.cpp:
* css/PropertySetCSSStyleDeclaration.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148359 268f45cc-cd09-0410-ab3c-d52691b4dbfc