Revert "Allow overriding self-update target file with envvar COMPOSER_SELF_UPDATE_TARGET"

Revert "Add docs for COMPOSER_SELF_UPDATE_TARGET, refs #8151"

This reverts commit e7eecc6901698ea352bff11ccabe5a1b6a6122c6.
This reverts commit faa7c5eea2de7e053f38c0726c01147531687361.

doc/03-cli.md

@@ -917,10 +917,6 @@ if you use Composer as super user at all times like in docker containers.
 
 If set, the value is used as php's memory_limit.
 
-### COMPOSER_SELF_UPDATE_TARGET
-
-If set, makes the self-update command write the new Composer phar file into that path instead of overwriting itself. Useful for updating Composer on read-only filesystem.
-
 ### COMPOSER_MIRROR_PATH_REPOS
 
 If set to 1, this env changes the default path repository strategy to `mirror` instead

src/Composer/Command/SelfUpdateCommand.php

@@ -351,10 +351,6 @@ TAGSPUBKEY
                 @copy($localFilename, $backupTarget);
             }
 
-            if ($targetFilename = getenv('COMPOSER_SELF_UPDATE_TARGET')) {
-                $localFilename = realpath($targetFilename) ?: $targetFilename;
-            }
-
             rename($newFilename, $localFilename);
 
             return null;