RFR: 8284775: Simplify String.substring(_, length()) calls

[Andrey Turbanov]

Default `to` index for String.substring method call is length of the String.
We can simplify code a bit, by omitting default value.

-------------

Commit messages:
 - [PATCH] Unnecessary string length argument for String.substring in java.desktop
 - [PATCH] Unnecessary string length argument for String.substring in java.desktop

Changes: https://git.openjdk.java.net/jdk/pull/8212/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8212&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284775
  Stats: 19 lines in 6 files changed: 0 ins; 5 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8212.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8212/head:pull/8212

PR: https://git.openjdk.java.net/jdk/pull/8212