Remove unused BUILTIN_PROXY_STACKSIZE config

It has been unused since the following change:
"All argv types should be char * const * not const char **"

builtin/Kconfig

@@ -1,18 +0,0 @@
-#
-# For a description of the syntax of this configuration file,
-# see the file kconfig-language.txt in the NuttX tools repository.
-#
-
-menu "Built-In Applications"
-	depends on BUILTIN
-
-config BUILTIN_PROXY_STACKSIZE
-	int "Builtin Proxy Stack Size"
-	default 1024
-	---help---
-		If exec_builtin uses I/O redirection options, then it will require
-		an intermediary/proxy task to muck with the file descriptors.  This
-		configuration item specifies the stack size used for the proxy. Default:
-		1024 bytes.
-
-endmenu # Built-In Applications