Remove unnecessary volatiles from volk_32fc_s32f_magnitude_16i

[argilo]

Several variables in volk_32fc_s32f_magnitude_16i were marked as volatile in an attempt to resolve a flaky test:

#282 (comment)
#282 (comment)
#285

These changes did not actually resolve the flaky test; it still failed on i386, and the test itself was later fixed in #713. Therefore, we can now revert the additions of volatile that were made in #285.

[argilo]

Removing the volatiles speeds up the generic protokernel significantly. The sse protokernel also sees a small improvement.

Before:

RUN_VOLK_TESTS: volk_32fc_s32f_magnitude_16i(131071,1987)
generic completed in 757.907 ms
a_avx2 completed in 75.477 ms
a_sse3 completed in 432.871 ms
a_sse completed in 466.542 ms
u_avx2 completed in 76.9684 ms

After:

RUN_VOLK_TESTS: volk_32fc_s32f_magnitude_16i(131071,1987)
generic completed in 471.106 ms
a_avx2 completed in 77.2529 ms
a_sse3 completed in 417.156 ms
a_sse completed in 399.88 ms
u_avx2 completed in 76.719 ms

[jdemel]

LGTM