Block schema has two uuid keys

The commit introduced in #1964254: Configuration schemas missing langcode and uuid at places added an extra uuid key in block.schema.yml. This does not have any effect on anything, but should be cleaned up.

Propose to remove the uuid key below label_display.

Git blame:

b92c56b1 (Alex Pott 2013-04-10 09:40:30 +0100 10)     uuid:
b92c56b1 (Alex Pott 2013-04-10 09:40:30 +0100 11)       type: string
b92c56b1 (Alex Pott 2013-04-10 09:40:30 +0100 12)       label: 'UUID'
3b449413 (webchick  2013-03-29 10:55:22 -0700 13)     label:
3b449413 (webchick  2013-03-29 10:55:22 -0700 14)       type: label
3b449413 (webchick  2013-03-29 10:55:22 -0700 15)       label: 'Title'
3b449413 (webchick  2013-03-29 10:55:22 -0700 16)     label_display:
3b449413 (webchick  2013-03-29 10:55:22 -0700 17)       type: string
3b449413 (webchick  2013-03-29 10:55:22 -0700 18)       label: 'Display title'
3b449413 (webchick  2013-03-29 10:55:22 -0700 19)     uuid:
3b449413 (webchick  2013-03-29 10:55:22 -0700 20)       type: string
3b449413 (webchick  2013-03-29 10:55:22 -0700 21)       label: 'UUID'