Remove the WTF::KeyValuePair default constructor

SSIA. This constructor is never used, besides it
leaves the pair elements uninitialized which
can cause bugs if it is used at some point in future.

Review URL: https://codereview.chromium.org/573203002

git-svn-id: svn://svn.chromium.org/blink/trunk@182205 bbb929c8-8fbe-4397-9dbb-9b2b20218538