RFR: 8293879: Remove unnecessary castings in jdk.hotspot.agent

[Andrey Turbanov]

Redundant castings make code harder to read.
Found them by IntelliJ IDEA.
I tried to choose only casts which are definitely safe to remove.
Most generification was done in [JDK-8241618](https://bugs.openjdk.org/browse/JDK-8241618), but casts weren't removed.

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Commit messages:
 - [PATCH] Remove unnecessary castings in jdk.hotspot.agent

Changes: https://git.openjdk.org/jdk/pull/10295/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10295&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293879
  Stats: 261 lines in 75 files changed: 0 ins; 8 del; 253 mod
  Patch: https://git.openjdk.org/jdk/pull/10295.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10295/head:pull/10295

PR: https://git.openjdk.org/jdk/pull/10295