MDL-44364 theme_bootstrapbase: Clean up calendar variables

There are a few reasons why I am cleaning up the calendar.less file.

1/ The variables were too specific, and unlikely to be overridden. If
   you are going to change the colour of the background, surely you will
   change the border to match the background. And if you want to do
   more advanced things, you will override the selectors anyway.

2/ If you wanted to re-use the color of the event, but not as a
   background or border, you would have to use a variable that
   was specific to the background or the border, a generic
   variable for the event colour seems required.

3/ The colour @redDarker should not be defined there, and its purpose
   is so limited that I do not think we need it at all. Also,
   this variable is unlikely to be re-used ever, better not
   introducing it.

4/ I removed the calendarHeadingsColor because those headings should
   really match the rest of the headings throughout the site. I
   removed it from the overlay, but used the standard Bootstrap
   one for the one in the page.

There are still a few things that would need improvement, like the
usage of @white for the background of the event, which will not really
look good on a dark background.