rust: build_error: remove note on optimization

Since commit fee989b ("rust: drop support for different optimization
levels than C") those optimization levels do not exist anymore, thus drop
the note about them.

Fixes: fee989b ("rust: drop support for different optimization levels than C")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

lib/Kconfig.debug

@@ -2812,8 +2812,6 @@ choice
 	  You can choose to abort compilation or ignore them during build and let the
 	  check be carried to runtime.
 
-	  If optimizations are turned off, you cannot select "Deny".
-
 	  If unsure, say "Deny".
 
 config RUST_BUILD_ASSERT_ALLOW