Remove mitre from list

The MITRE Corporation is an important organization in the security
field. Having the word `mitre` included in this list prevents the
organization's name from being mentioned.

doc/.vale/gitlab/British.yml

@@ -67,7 +67,6 @@ swap:
   matt: matte
   meagre: meager
   metre: meter
-  mitre: miter
   modelling: modeling
   moustache: mustache
   neighbour: neighbor