CI remove redundant checks covered by pyupgrade

[MarcoGorelli]

Some of these checks, e.g.

    MSG='Check for python2-style file encodings' ; echo $MSG
    invgrep -R --include="*.py" --include="*.pyx" -E "# -\*- coding: utf-8 -\*-" pandas scripts
    RET=$(($RET + $?)) ; echo $MSG "DONE"

    MSG='Check for python2-style super usage' ; echo $MSG
    invgrep -R --include="*.py" -E "super\(\w*, (self|cls)\)" pandas
    RET=$(($RET + $?)) ; echo $MSG "DONE"

are already checked by pyupgrade.

Opening this as a placeholder to go through the checks and see what no longer needs to be there

EDIT

also, as a note, in pandas/core/generic.py there is an instance of .. warning:, but the Check for incorrect sphinx directives isn't picking it up. pygrep finds it though. I'm inclined to move all of these over to pre-commit so that they're cross-platform and can selectively be run only on staged files