[TASK] Crisp up navigation frames borders

Since UXW 2009 module navigation and tree frame have
soft border / gradient shadow on their right.

On or path to flat backend:
bring super crisp borders.

Also removes vertical gradient behind module menu bar.

Change-Id: I826df77447d782c85f1c08440632ee438c3c37b0
Resolves: #48586
Releases: 6.2
Reviewed-on: https://review.typo3.org/21125
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn