docs: fs: fscrypt.rst: get rid of :c:type: tags

The :c:type: tag has problems with Sphinx 3.x, as structs
there should be declared with c:struct.

So, remove them, relying at automarkup.py extension to
convert them into cross-references.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>