Remove the deprecated legacy slash commands (#1188)

commands/brainstorm.md

@@ -1,5 +0,0 @@
----
-description: "Deprecated - use the superpowers:brainstorming skill instead"
----
-
-Tell your human partner that this command is deprecated and will be removed in the next major release. They should ask you to use the "superpowers brainstorming" skill instead.

commands/execute-plan.md

@@ -1,5 +0,0 @@
----
-description: "Deprecated - use the superpowers:executing-plans skill instead"
----
-
-Tell your human partner that this command is deprecated and will be removed in the next major release. They should ask you to use the "superpowers executing-plans" skill instead.

commands/write-plan.md

@@ -1,5 +0,0 @@
----
-description: "Deprecated - use the superpowers:writing-plans skill instead"
----
-
-Tell your human partner that this command is deprecated and will be removed in the next major release. They should ask you to use the "superpowers writing-plans" skill instead.