Removing default usages of the PERC2 and TAB2 clefs.

- PERC2 is no longer supported and always rendered as PERC; using it as default, as in importxml.cpp is obsolete.
- TAB is the default tab clef, and TAB2 only remains as a stylistic variant.
Fix Guitar Pro tests
Merge pull request #939 from mgavioli/Phasing_out_PERC2_and_TAB2_clefs

Removing default usages of the PERC2 and TAB2 clefs.