remove incorrect reference

the semantics are not explained in the referenced section any more, they
have been moved to the documentation for common options in the new CLI [0].

[0]: 703d863

doc/manual/src/command-ref/opt-common.md

@@ -203,10 +203,9 @@ Most Nix commands accept the following command-line options:
     instead.
 
   - <span id="opt-I">[`-I`](#opt-I)</span> *path*\
-    Add a path to the Nix expression search path. This option may be
-    given multiple times. See the `NIX_PATH` environment variable for
-    information on the semantics of the Nix search path. Paths added
-    through `-I` take precedence over `NIX_PATH`.
+    Add a path to the Nix expression search path.
+    This option may be given multiple times.
+    Paths added through `-I` take precedence over [`NIX_PATH`](./env-common.md#env-NIX_PATH).
 
   - <span id="opt-option">[`--option`](#opt-option)</span> *name* *value*\
     Set the Nix configuration option *name* to *value*. This overrides