[FIX] undo check for TIC

TIC does not need to be present in a valid mzML, requiring it to be
present in all cases causes many valid mzML files to not be parsed at
all.

undo commit ef404d671b0afc6dc03be4d03715fd068de54f96
Merge pull request #16 from hroest/TICfix

[FIX] undo check for TIC