Remove SCR2

... as SCR2 is redundant.

SCR20 provides more generalized and comprehensive information. We should remove SCR2.

SCR2: Using redundant keyboard and mouse event handlers
http://www.w3.org/WAI/GL/2014/WD-WCAG20-TECHS-20140724/SCR2.html

SCR20: Using both keyboard and other device-specific functions
http://www.w3.org/WAI/GL/2014/WD-WCAG20-TECHS-20140724/SCR20.html