arm64/sme: Drop inaccurate documentation of streaming mode switches

The SME ABI documentation contains an inaccurate description of the
architectural streaming mode entry/exit behaviour, just remove it since
this is better documented by the architecture or with the rest of the
documentation for the specific software interfaces concerned.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>