README.md: remove old warning about ext4 encryption on removable media

This warning is only applicable for systems with kernel versions older
than v4.1, which is now too old to be worth the mention here.

README.md

@@ -305,10 +305,6 @@ It is also recommended to use Linux kernel v5.4 or later, since this
 allows the use of v2 encryption policies.  v2 policies have several
 security and usability improvements over v1 policies.
 
-Be careful when using ext4 encryption on removable media, since ext4 filesystems
-with the `encrypt` feature cannot be mounted on systems with kernel versions
-older than the minimums listed above -- even to access unencrypted files!
-
 If you configure `fscrypt` to use non-default features, other kernel
 prerequisites may be needed too.  See [Configuration
 file](#configuration-file).