Minor: delete PlainText implementation

- It never gets used anymore and could cause security issues if we did accidentally use it as EncryptionMethod

src/main/java/fr/xephi/authme/security/HashAlgorithm.java

@@ -34,7 +34,7 @@ public enum HashAlgorithm {
 
     @Deprecated DOUBLEMD5(fr.xephi.authme.security.crypts.DoubleMd5.class),
     @Deprecated MD5(fr.xephi.authme.security.crypts.Md5.class),
-    @Deprecated PLAINTEXT(fr.xephi.authme.security.crypts.PlainText.class),
+    @Deprecated PLAINTEXT(null),
     @Deprecated SHA1(fr.xephi.authme.security.crypts.Sha1.class),
     @Deprecated SHA512(fr.xephi.authme.security.crypts.Sha512.class),
     @Deprecated WHIRLPOOL(fr.xephi.authme.security.crypts.Whirlpool.class);

src/main/java/fr/xephi/authme/security/crypts/PlainText.java

@@ -1,20 +0,0 @@
-package fr.xephi.authme.security.crypts;
-
-import fr.xephi.authme.security.crypts.description.Recommendation;
-import fr.xephi.authme.security.crypts.description.Usage;
-
-/**
- * Plaintext password storage.
- *
- * @deprecated Using this is no longer supported. AuthMe will migrate to SHA256 on startup.
- */
-@Deprecated
-@Recommendation(Usage.DEPRECATED)
-public class PlainText extends UnsaltedMethod {
-
-    @Override
-    public String computeHash(String password) {
-        return password;
-    }
-
-}