[BUGFIX] Fix RewriteRule for context to avoid HTTP 500

The affected rules are those for TYPO3_CONTEXT.
These rules should just set the ENV variable,
but should not touch the URL.
According to apache httpd docs this has to be
indicated by using a dash.

Also unnecessary grouping in the regexp has been
removed.

Resolves: #59037
Releases: 6.3, 6.2
Change-Id: I56cadcfb3cfae0a0ee679c6886cda9f5498fc47c
Reviewed-on: https://review.typo3.org/30328
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Armin Ruediger Vieweg
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel