[FIX] sap.m.MessageBox: HCB theme clean up

Cleaning up the css styles according the new implementation
specification; The styles in the MessageBox are absolete and only
override already existing styles that corespond to the required look and
feel;

Change-Id: Iabc5aed567860089ee12120751c5832254ecfe0d
Merge "[FIX] sap.m.MessageBox: HCB theme clean up"