[IMPR] remove old 'blockpageschecker-summary' key
[IMPR] remove old 'blockpageschecker-summary' key

After 'blockpageschecker-summary' was renamed, the fallback implementation
can eb removed.

Change-Id: I2d93bc698e46c01656f6f94da26b21229284e117

wikimedia/pywikibot-core,wikimedia/pywikibot-core