[SYCL][NFC] Clean up __imf_* delarations in sycl intel math header

[jinge90]

There are duplicates for _imf* declarations in sycl/builtins.hpp, this PR aims to remove them, the necessary declarations will be moved into sycl/ext/intel/math.hpp to make sycl builtins.hpp clean.
We also unify the style for _imf* declarations in sycl intel math header to move them close to where they are invoked.

[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.