tests: cmsis_dsp: transform: Remove MPS2 AN521 from intg. platforms

This commit removes the `mps2_an521_remote` board from the integration
platform list of the CMSIS-DSP transform tests because this board no
longer has a sufficient code memory (FLASH) to fit these tests after
the resizing done in the PR #52052 for TF-M compatibility.

Consider adding `mps2_an386` as an integration platform for these tests
once the support for it is added to Zephyr (refer to the issue #45319).

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>