Remove unused tag manager files

In the (unfinished) process of trying to understand the tag manager code, I noticed that tm_project and tm_symbol aren't used anywhere in the code. The only place tm_project is used is some dumping code but since it's not created anywhere, that code is clearly dead.

The patch removes tm_project and tm_symbol and also docstrings referring to them. I believe it doesn't make sense to keep unmaintained files which aren't used (over -1000LOC). In addition, I removed the empty tm_tagmanager.c file (not sure if there's some reason behind keeping it, e.g. docstring generation orwhatever).

While going through the tag manager code, I've actually noticed quite a large amount of code which is never executed. My question is: can I do some additional cleanup in tag manager and remove functions/code paths which are never executed in Geany? Or are there some other plans with the TM code?