Amending Base10 to slightly darker `#202122` to fulfill WCAG
requirements in connection to Accent50.
Also adding it as WikimediaUI Base equivalent variable for
future replacement.
Also removing variables not in use any more.
Bug: T248393
Change-Id: I6fb5c288cfeb2bd3c786c2f5ab771074203b8fc0
Renaming 'mmv.globals.less' to 'mmv.variables.less' following naming
pattern of other extensions like MobileFrontend, Echo or Flow.
Change-Id: I376c3fd1463ed225e214f165943576f3e3c7df39