A little cleanup for Undertale music

Please delete Gatser’s Theme, as the name is misspelled and I’ve made a better transcription under the real name, and the Chorus in His Theme, as it’s the same melody as the Verse, but looped, and with less accurate and more complicated chords. Also, please move all of the Toby Fox songs whose names contain "Undertale - " to unprefixed versions, for clarity.

I deleted that tab and removed all the prefixes on every song I with undertale in the name. Thanks!