misc: atmel: Remove CPU_AT32AP700X (AVR32) reference

The CPU_AT32AP700X symbol symbol went away when when AVR32 was removed
in commit 26202873 ("avr32: remove support for AVR32 architecture")

Remove the prompt from ATMEL_TCB_CLKSRC_BLOCK. The prompt condition
could never be satisfied now.

Discovered with the
https://github.com/ulfalizer/Kconfiglib/blob/master/examples/list_undefined.py


script.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>