[SYCL][NFC] Remove redundant __imf_fsigm* declarations

[jinge90]

We have declared __imf_fsigm* in https://github.com/intel/llvm/pull/20873/files#diff-8da594ced8852cd2e62e9d1fcd013ca0b2256b88b3745b30a239bb01aa7b2011R251
So, the declarations at the begining is redundant, just remove them.

[bader]

We have declared __imf_fsigm* in https://github.com/intel/llvm/pull/20873/files#diff-8da594ced8852cd2e62e9d1fcd013ca0b2256b88b3745b30a239bb01aa7b2011R251
So, the declarations at the begining is redundant, just remove them.

I suggest we keep the first declaration and remove the second instead.
It keeps sigmoid declarations aligned with the rest of the declarations.