[Linking] Remove 'Required' (0x3d) prefix from features section (#236)

This has not been emitted in LLVM since
llvm/llvm-project@3f34e1b.

Author: aheejin

Linking.md

@@ -402,15 +402,14 @@ The contents of the target features section is a vector of entries:
 
 The recognized prefix bytes and their meanings are below. When the user does not
 supply a set of allowed features explicitly, the set of allowed features is
-taken to be the set of features used or required. Any feature not mentioned in
-an object's target features section is not used by that object, but is not
-necessarily prohibited in the final binary.
+taken to be the set of used features. Any feature not mentioned in an object's
+target features section is not used by that object, but is not necessarily
+prohibited in the final binary.
 
 | Prefix     | Meaning |
 | ---------- | ------- |
 | 0x2b (`+`) | This object uses this feature, and the link fails if this feature is not in the allowed set. |
 | 0x2d (`-`) | This object does not use this feature, and the link fails if this feature is in the allowed set. |
-| 0x3d (`=`) | This object uses this feature, and the link fails if this feature is not in the allowed set or if any object does not use this feature. |
 
 The generally accepted features are: