[RFC] remove non-VECTOR code path in nnue_accumulator.cpp

[ces42]

this is a PR on top of #7013.
This might be a slight slowdown for binaries without vectorization. E.g. x86-32 running on Redwood Cove:

Result of 80 runs (counting nodes/1B cycles)
=============================================
base (...ckfish.mmx.orig) =   37369 +/- 1136
test (...sh.mmx.tile1024) =   37039 +/- 1109
speedup %                 =   -0.85 +/- 0.08
                             [95% CI; t-test]

likelihood(speedup > 0)   =  0.0000

CPU: 16 x Intel(R) Core(TM) Ultra 9 185H

Should result in exactly the same code after pre-processing for any uarch that has defined(VECTOR) || defined(USE_RVV).

No functional change

[github-actions]

clang-format 20 needs to be run on this PR.
If you do not have clang-format installed, the maintainer will run it when merging.
For the exact version please see https://packages.ubuntu.com/questing/clang-format-20.
An easier way to install it might be through https://apt.llvm.org/#llvmsh.

(execution 30462432105 / attempt 1)

[ces42]

(this is currently broken on RISC-V)

[anematode]

I think this is good, the non-vector impls should just be there for correctness purposes

[Torom]

base: #7013 test: #7014

sf_base =    2259612 +/-    1863 (95%)
sf_test =    2259660 +/-    1820 (95%)
diff    =         47 +/-    2622 (95%)
speedup =   0.00211% +/-  0.116% (95%)

Probability candidate is faster than baseline by at least
   0.00%:  51.42%
   0.25%:   0.00%
   0.50%:   0.00%
   1.00%:   0.00%