Optimise method exec in TQSlot and revert opt in TSAXParser

This is a combination of 2 commits in 1 to avoid incomplete
clones/checkouts.

Revert "Solve ROOT-6591: SAX parser much faster now."

This reverts commit c3ed90cfa8f49bed3b1ae31a8a4318559a75ba44.
The optimisation is now upstream in the call stack in TQSlot.

Optimise the method execution, reduce amount of code, adjust indentation.
Coding conventions

while studying this code I realised it does not respect at all
the coding conventions.