[DOC] Remove `any` flag

[wdevazelhes]

In the end the any flag set here:

metric-learn/doc/conf.py

Line 42 in 66a12ed

default_role = 'any'

may cause more harm than good, because it displays a lot of errors when building the doc and sometimes it's hard to find true errors, so in the end we should maybe come back to a manual linking in the doc

[mvargas33]

I've been having a lot of trouble as well, because there are sooo many warnings due to this that it's difficult to debug the current changes in the docs.