bw.transform - remove deprecation statements needed for Pandas backward compatibility

[stephenholleran]

Remove global variables and deprecation statements that were used to make the brightwind library backward compatible for versions of Pandas before v2.2.0. These are to do with frequency strings Pandas uses to do time offsets.

Remove the global variables:

And remove the ability for the brightwind library to handle freq strings of 'A', 'AS', 'H', 'T' and 'S'.