[sw, rom_ext_signer] Remove toochains file

Cargo does not like empty toolchains file, and won't do `Cargo fmt`
and `Cargo test` unless it is removed.

If we need it back at some point there is a way in meson to check
whether file exists, but for now it is removed.

Signed-off-by: Silvestrs Timofejevs <silvestrst@lowrisc.org>
diff --git a/sw/host/rom_ext_image_tools/rust-toolchain b/sw/host/rom_ext_image_tools/rust-toolchain
deleted file mode 100644
index e69de29..0000000
--- a/sw/host/rom_ext_image_tools/rust-toolchain
+++ /dev/null
diff --git a/sw/host/rom_ext_image_tools/signer/meson.build b/sw/host/rom_ext_image_tools/signer/meson.build
index 4c1db78..0a00996 100644
--- a/sw/host/rom_ext_image_tools/signer/meson.build
+++ b/sw/host/rom_ext_image_tools/signer/meson.build
@@ -7,7 +7,6 @@
 build_type = 'release'
 build_dir = meson.current_build_dir()
 manifest_path = meson.current_source_dir() / 'Cargo.toml'
-toolchain_file = meson.current_source_dir() / '..' / 'rust-toolchain'
 
 # CARGO FLAGS:
 # These flags will only apply to the final binary, and won't get propogated
@@ -39,14 +38,13 @@
     cargo,
     cargo_flags,
     rust_flags,
-    toolchain_file,
+    '',
     meson.source_root(),
     meson.build_root(),
   ],
   depend_files: [
     cargo_invoke_cmd,
     manifest_path,
-    toolchain_file,
   ],
   output: '.',
   console: true,