Remove stale reference to removed folly SplitStringSimd.cpp

[rootkiller6788]

Summary

Closes #242.

CMakeLists.txt listed ${FOLLY_SOURCE_DIR}/folly/detail/SplitStringSimd.cpp in the FOLLY_CPP_SRC set, but folly removed that translation unit on 2025-10-27 (fbcode D85101083, "allow all container and string types in split()").

The split() implementation moved into headers: folly/detail/SplitStringSimd.h now includes folly/detail/SplitStringSimdImpl.h, which is fully header-only (all templates, FOLLY_NOINLINE/FOLLY_ALWAYS_INLINE). No link-time symbol from a .cpp is needed by wdt's use of folly::split (e.g. wdtCmdLine.cpp splits on '\t' via the header-only simdSplitByChar).

Keeping the stale entry makes CMake fail at configure time against a current folly checkout:

CMake Error at CMakeLists.txt:79: Cannot find source file:
  .../folly/detail/SplitStringSimd.cpp

This removes the stale entry. All other folly sources in FOLLY_CPP_SRC still exist in folly main (verified against the folly git tree).

Verification

• Confirmed folly/detail/SplitStringSimd.cpp is absent from folly's current main tree (404 via the contents API), while SplitStringSimd.h / SplitStringSimdImpl.h remain.
• Confirmed all other 9 folly .cpp files in FOLLY_CPP_SRC exist in folly main.
• Confirmed wdt's only use of char-splitting (folly::split('\t', ...) in wdtCmdLine.cpp) resolves via the header-only implementation.

Test plan

N/A — CMake source-list change; configure succeeds once the non-existent file is removed.