Bug #57338: Extreneous server variable thread_pool_size

The server contained code for the server variable and
option thread_pool_size, but this server variable where
not used anywhere.

The variable is probably remains from backporting too
much from 6.0 (specifically, the thread pool
implementation was not backported from 6.0, which this
variable is associated with).

This patch eliminates the variable from the server.

sql/sys_vars.cc

@@ -2014,15 +2014,6 @@ static Sys_var_ulong Sys_thread_cache_size(
        GLOBAL_VAR(thread_cache_size), CMD_LINE(REQUIRED_ARG),
        VALID_RANGE(0, 16384), DEFAULT(0), BLOCK_SIZE(1));
 
-#if HAVE_POOL_OF_THREADS == 1
-static Sys_var_ulong Sys_thread_pool_size(
-       "thread_pool_size",
-       "How many threads we should create to handle query requests in "
-       "case of 'thread_handling=pool-of-threads'",
-       GLOBAL_VAR(thread_pool_size), CMD_LINE(REQUIRED_ARG),
-       VALID_RANGE(1, 16384), DEFAULT(20), BLOCK_SIZE(0));
-#endif
-
 /**
   Can't change the 'next' tx_isolation if we are already in a
   transaction.