Remove incorrect rules from trait names proposal

The proposal currently adds some rules to trait names. However some of those rules conflict with the statement that all trait names must be valid Swift identifiers. So let's remove those rules.

Author: FranzBusch

proposals/0450-swiftpm-package-traits.md

@@ -512,14 +512,8 @@ Since traits can show up both in the `Package.swift` and in source code when
 checking if a trait is enabled, the allowed characters for a trait name are
 restricted to [legal Swift
 identifier](https://docs.swift.org/swift-book/documentation/the-swift-programming-language/summaryofthegrammar/).
-Hence, the following rules are enforced on trait names:
-
-- The first character must be a [Unicode XID start
-  character](https://unicode.org/reports/tr31/#Figure_Code_Point_Categories_for_Identifier_Parsing)
-  (most letters), a digit, or `_`.
-- Subsequent characters must be a [Unicode XID continue
-  character](https://unicode.org/reports/tr31/#Figure_Code_Point_Categories_for_Identifier_Parsing)
-  (a digit, `_`, or most letters), `-`, or `+`.
+Additional, the following rules are enforced on trait names:
+
 - `default` and `defaults` (in any letter casing combination) are not allowed as
   trait names to avoid confusion with default traits.