Remove square brackets from key signature facet values

[ahankinson]

The supplied values in the key signatures mean that key signatures that are the same are split across many different values. To find all the incipits that are in Ab, the user needs to select all of these:

• b[B]EAD
• bBEAD
• bBEA[D]
• bBE[AD]
• bB[E]AD
• b[B]EA[D]
• bB[EAD]
• bBE[A]D

For the purposes of the facet, we should remove all the square brackets in the key signature, so that users have only one value to select, instead of 8!