Add function to remove protocols declared but not used

[agbeltran]

This is not critical as it would be extra information in the ISA files (and the validator only gives a warning), but within the set of curation functions, we can consider adding one to remove those protocols declarations that are never used.

An example warning in this case is:

WARNING:isatools.isatab:(W) Some protocols declared in the file s_gsk_longitudinal.txt are not used in any assay file: ['', 'data transformation', 'extraction', 'labeling']