Don't look for gprbuild from system provider.

Because the manifest currently asks for Alire to attempt to download
gprbuild from the system package manager, alr <anything> spends
several fruitless seconds trying to find gprbuild.

  * index/gp/gprbuild/gprbuild-external.toml: remove the 'system' external.

index/gp/gprbuild/gprbuild-external.toml

@@ -9,6 +9,3 @@ kind = "version-output"
 version-regexp = "^GPRBUILD ([\\d\\.-]+).*|^GPRBUILD Community ([\\d\\.-]+).*"
 version-command = ["gprbuild", "--version"]
 
-[[external]]
-kind = "system"
-origin = ["gprbuild"]