Remove incorrect DCHECK in ~BrowserCdmManager.
Remove incorrect DCHECK in ~BrowserCdmManager.

https://codereview.chromium.org/1225083002 fixed the core issue but left an
incorrect DCHECK in the code. It is not the case that only a single
BrowserCdmManager per RPH ID may be alive at a time, only that a single one
corresponds to a given live process. (A reference-counted type can never be
assumed to have bounded lifetime for the same reason garbage-collected types
can never be assumed to have bounded lifetime.)

This fixes ChildProcessLauncherBrowserTest.ChildSpawnFail's flakiness on
cast_shell_linux when use_openssl is switched.

BUG=506188,462040

Review URL: https://codereview.chromium.org/1235653002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#338312}

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