[BUGFIX] Remove L param. from preview URL when no language is specified.

It helps in generation of clean speaking URLs, especially when system
uses only single or default language.

Change-Id: I3733dce7a9061c6b0afab0ecf8ffc96461b517c2
Reviewed-on: https://review.typo3.org/24856
Reviewed-by: Chetan Thapliyal
Tested-by: Chetan Thapliyal
[TASK] Fix deprecated `t2lib_BEfunc::compilePreviewKeyword` function.

Change-Id: I79d3fa495d0232e8af1cc4a27ef0020ea075fb1b
Reviewed-on: https://review.typo3.org/24857
Reviewed-by: Chetan Thapliyal
Tested-by: Chetan Thapliyal