Remove validation for custom toolchains when reading rust-toolchain.toml

This validation was introduced in e1306b3
However, it breaks a number of scenarios that previously worked without
issue such as #4248. As requested by rustup maintainers in that thread,
I'm taking out the extra validation added in that commit, leaving the
rest of the code as-is.

I was not able to figure out a way to write an automated test for this
scenario since the rustup CLI needs the custom toolchain to actually
exist and unit testing looked infeasible due to use of the `Cfg` type.

Manual testing confirms this fixes #4248.

Author: wesleywiser

Diff for: src/config.rs

@@ -181,18 +181,7 @@ impl OverrideCfg {
                         .transpose()?,
                 }
             }
-            ToolchainName::Custom(name) => {
-                if file.toolchain.targets.is_some()
-                    || file.toolchain.components.is_some()
-                    || file.toolchain.profile.is_some()
-                {
-                    bail!(
-                        "toolchain options are ignored for a custom toolchain ({})",
-                        name
-                    )
-                }
-                Self::Custom(name)
-            }
+            ToolchainName::Custom(name) => Self::Custom(name),
         })
     }