ATLASRECTS-5201: Remove ftagfloat_t

We introduced ftagfloat_t a while ago to try to keep code similar between r21 and r22: a lot of variables were double in r21 and we moved them to floats in r22.

At this point, however, there is very little reason to touch core r21 flavor tagging code, and the two releases have diverged quite a bit in other ways. We might as well get rid of this variable, which is sort of confusing anyway.

Marking as a draft for now: this MR took me all of 5 minutes to prepare and we can do the same thing at any point in the future, so it's not urgent.

Steps to reproduce:

# remove the header file
rm xAODBTagging/ftagfloat_t.h
# remove the lines where this file is included
git grep -l 'ftagfloat_t\.h' | while read x ; do sed '/#include.*ftagfloat_t/d' $x > /tmp/file ; mv /tmp/file $x ; done
# change the `ftagfloat_t` to `float`
git grep -l 'ftagfloat_t' | while read x ; do sed 's/ftagfloat_t/float/g' $x > /tmp/file ; mv /tmp/file $x ; done