Removed workarounds for old gcc's

Removed gcc 2 workaround, as there are no other comparisons
of __GNUC__ against the number 2 (except KWSys and 3rd party).
Removed broken VTK_CXX_GCC_3 since __GNUC__ can't be equal to
and less than 3 at the same time.

Change-Id: Iefdbb1a417fac120ce74ed91d030e4f7dbcdfd0f
Merge topic 'purge-old-gcc-crud' into master

214c83d3 Removed workarounds for old gcc's