Help: Fix file(INSTALL) docs w.r.t. CMAKE_INSTALL_MESSAGE

In commit c9568de52c (install: Add CMAKE_INSTALL_MESSAGE variable
(#13761), 2014-06-24, v3.1.0-rc1~370^2~1) we incorrectly documented
that `CMAKE_INSTALL_MESSAGE` controls the status message for
`file(INSTALL)`.  Revert that.

Fixes: #17162

Help/command/file.rst

@@ -838,8 +838,8 @@ of their content even if options are used to select a subset of
 files.
 
 The ``INSTALL`` signature differs slightly from ``COPY``: it prints
-status messages (subject to the :variable:`CMAKE_INSTALL_MESSAGE` variable),
-and ``NO_SOURCE_PERMISSIONS`` is default.
+status messages, and ``NO_SOURCE_PERMISSIONS`` is default.
+
 Installation scripts generated by the :command:`install` command
 use this signature (with some undocumented options for internal use).