Remove the left-over & unused code for supporting "Scroll Lock" key

It is decided to not reimplement this feature in KDE4 konsole:

  * It is non-trivial work to implement it correctly
  * It annoyed some users when it was available in KDE3 konsole
  * Not many users want this feature back

This is a follow up of commit cee0ce53

CCBUG:172271
REVIEW:104156