wasi-nn: Support multiple TFLite models (#2002)

Remove restrictions:
- Only 1 WASM app at a time
- Only 1 model at a time
   - `graph` and `graph-execution-context` are ignored

Refer to previous document:
e8d718096d/core/iwasm/libraries/wasi-nn/README.md

core/iwasm/libraries/wasi-nn/wasi_nn.cmake

@@ -3,8 +3,6 @@
 
 set (WASI_NN_DIR ${CMAKE_CURRENT_LIST_DIR})
 
-add_definitions (-DWASM_ENABLE_WASI_NN=1)
-
 include_directories (${WASI_NN_DIR})
 include_directories (${WASI_NN_DIR}/src)
 include_directories (${WASI_NN_DIR}/src/utils)