Revert "magic pathspec: add ":(icase)path" to match case insensitively"

This reverts commit d0546e2d48, which
was only meant to be a Proof-of-concept used during the discussion.

The real implementation of the feature needs to wait until we migrate
all the code to use "struct pathspec", not "char **", to represent
richer semantics given to pathspec.

Documentation/glossary-content.txt

@@ -319,13 +319,10 @@ top `/`;;
 	The magic word `top` (mnemonic: `/`) makes the pattern match
 	from the root of the working tree, even when you are running
 	the command from inside a subdirectory.
-icase;;
-	The magic word `icase` (there is no mnemonic for it) makes the
-	pattern match case insensitively.  E.g. `:(icase)makefile` matches
-	both `Makefile` and `makefile`.
 --
 +
-It is envisioned that we will support more types of magic in later
+Currently only the slash `/` is recognized as the "magic signature",
+but it is envisioned that we will support more types of magic in later
 versions of git.
 
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