Remove SSE protokernels written in assembly

[argilo]

Only three protokernels remain that are written entirely in assembly:

• volk_32fc_x2_conjugate_dot_prod_32fc_a_sse
• volk_32fc_x2_dot_prod_32fc_u_sse_64
• volk_32fc_x2_dot_prod_32fc_a_sse_64

These are holdovers from the early days of GNU Radio, and I don't think keeping them is worth the maintenance cost. Both volk_32fc_x2_conjugate_dot_prod_32fc and volk_32fc_x2_dot_prod_32fc have SSE3 protokernels which perform better, and SSE3 has been around since 2004.

[jdemel]

LGTM. I think we discussed expected CPU features lately. In essence, 32bit x86 is probably hard to find nowadays. It's reasonable to remove the maintenance burden here.
64bit x86 requires at least SSE2. A fact that caused issues in the past as well.