Disable stub warning by default.

When we create a class symbols from a classpath elements, references
to other classes that are absent from the classpath are represented
as references to "stub symbols". This is not a fatal error; for instance
if these references are from the signature of a method that isn't called
from the program being compiled, we don't need to know anything about them.
A subsequent attempt to look at the type of a stub symbols will trigger a
compile error.

Currently, the creation of a stub symbol incurs a warning. This commit
removes that warning on the basis that it isn't something users need
to worry about. javac doesn't emit a comparable warning.

The warning is still issued under any of `-verbose` / `-Xdev` / `-Ydebug`.

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