RFR: 8283254: Remove redundant class jdk/internal/agent/spi/AgentProvider

[Kevin Walls]

There are no uses of jdk/internal/agent/spi/AgentProvider, since the SNMP agent was removed ( 8071367 ): this class should be removed.  It is not a public interface.

Remove src/jdk.management.agent/share/classes/jdk/internal/agent/spi/AgentProvider.java
Remove import from src/jdk.management.agent/share/classes/jdk/internal/agent/Agent.java
Remove uses from src/jdk.management.agent/share/classes/module-info.java

Testing with the test/jdk/sun/management tests looks good.

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Commit messages:
 - 8283254: Remove redundant class jdk/internal/agent/spi/AgentProvider

Changes: https://git.openjdk.java.net/jdk/pull/7912/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7912&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283254
  Stats: 80 lines in 3 files changed: 0 ins; 79 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7912.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7912/head:pull/7912

PR: https://git.openjdk.java.net/jdk/pull/7912