chore: remove deprecated wasmedge_llmc plugin (#36)

Upstream deprecated the wasmedge_llmc plugin (WasmEdge/WasmEdge#4964).
Rather than migrate it into cpp-plugins, remove it: drop
plugins/wasmedge_llmc and test/plugins/wasmedge_llmc and their
add_subdirectory stanzas in plugins/CMakeLists.txt and
test/plugins/CMakeLists.txt. libpiper (wasi_nn) is unrelated and kept.

Closes #36

Signed-off-by: Yi Huang <yi@secondstate.io>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 0yi0

plugins/CMakeLists.txt

@@ -52,11 +52,6 @@ if(WASMEDGE_PLUGIN_IMAGE)
   endif()
 endif()
 
-# WasmEdge plug-in: LLMC.
-if(WASMEDGE_PLUGIN_LLMC)
-  add_subdirectory(wasmedge_llmc)
-endif()
-
 # WasmEdge plug-in: OCR.
 if(WASMEDGE_PLUGIN_OCR)
   add_subdirectory(wasmedge_ocr)