Remove deb Subcommand

[Alexhuszagh]

The subcommand deb runs in the container, however, cargo-deb is not installed by default. Since we don't support similar subcommands like asm, it is probably worth removing this subcommand until we support other subcommands (#716). The relevant diff would be:

diff --git a/src/cargo.rs b/src/cargo.rs
index 14e9585..b22d427 100644
--- a/src/cargo.rs
+++ b/src/cargo.rs
@@ -15,7 +15,6 @@ pub enum Subcommand {
     Rustc,
     Test,
     Bench,
-    Deb,
     Clippy,
     Metadata,
 }
@@ -44,7 +43,6 @@ impl<'a> From<&'a str> for Subcommand {
             "rustc" => Subcommand::Rustc,
             "t" | "test" => Subcommand::Test,
             "bench" => Subcommand::Bench,
-            "deb" => Subcommand::Deb,
             "clippy" => Subcommand::Clippy,
             "metadata" => Subcommand::Metadata,
             _ => Subcommand::Other,