tests: benchmarks: app_kernel: Remove k_mem_pool declaration

This commit removes the stale `k_mem_pool` declaration for the memory
pool API, which was removed in the v2.5.0 release.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

tests/benchmarks/app_kernel/src/master.h

@@ -147,8 +147,6 @@ extern struct k_pipe PIPE_BIGBUFF;
 
 extern struct k_mem_slab MAP1;
 
-extern struct k_mem_pool DEMOPOOL;
-
 
 
 /* PRINT_STRING