Revert "initrd: extend SYSTEMD_IN_INITRD to accept non-ramfs rootfs"

This reverts commit 1f22621ba3.

As described in the reverted commit, we don't want to get rid of the check
completely. But the check requires opting-in by setting SYSTEMD_IN_INITRD=lenient,
which is cumbersome and doesn't seem to actually happen.
https://bugzilla.redhat.com/show_bug.cgi?id=2137631 is caused by systemd refusing
to treat the system as an initrd because overlayfs is used. Let's revert this
approach and do something that doesn't require opt-in instead.

I don't think it makes sense to keep support for "SYSTEMD_IN_INITRD=lenient" or
"SYSTEMD_IN_INITRD=auto". To get "auto" behaviour, just unset the option. And
"lenient" will be reimplemented as a better check. Thus the changes to the
option interface are completely reverted.

docs/ENVIRONMENT.md

@@ -73,13 +73,9 @@ All tools:
   (relevant in particular for the system manager and `systemd-hostnamed`).
   Must be a valid hostname (either a single label or a FQDN).
 
-* `$SYSTEMD_IN_INITRD=[auto|lenient|0|1]` — if set, specifies initrd detection
-  method. Defaults to `auto`. Behavior is defined as follows:
-  `auto`: Checks if `/etc/initrd-release` exists, and a temporary fs is mounted
-          on `/`. If both conditions meet, then it's in initrd.
-  `lenient`: Similar to `auto`, but the rootfs check is skipped.
-  `0|1`: Simply overrides initrd detection. This is useful for debugging and
-         testing initrd-only programs in the main system.
+* `$SYSTEMD_IN_INITRD` — takes a boolean. If set, overrides initrd detection.
+  This is useful for debugging and testing initrd-only programs in the main
+  system.
 
 * `$SYSTEMD_BUS_TIMEOUT=SECS` — specifies the maximum time to wait for method call
   completion. If no time unit is specified, assumes seconds. The usual other units