Fix #11146: Don't re-add root imports to context in REPLFrontEnd

[griggt]

The root context for REPL compiler runs is initialized by ReplCompiler#newRun,
which contains special handling to:

1. first add the default root imports (java.lang._, scala._, Predef._)
2. then import each previous REPL wrapper in order (incl. its imports)

The result is the innermost context imports the most recent REPL wrapper
and the (nearly) outermost contexts contain the default imports.

Before this commit, the default root imports were also being added to the
(innermost) context in REPLFrontEnd#runOn, resulting in REPL definitions
being shadowed by those in java.lang._, scala._ and Predef._

The context before this PR as seen by the frontend would look something like the below (this is after two previous lines of REPL input.) Note the undesirable imports of java.lang._, scala._ and Predef._ in the innermost contexts.

Context(
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@619c93ca, import = import Predef.{...}, implicits = ...
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@619c93ca, import = import scala.{...}
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@619c93ca, import = import java.lang.{...}
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@619c93ca, import = import rs$line$2.{...}
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@619c93ca, import = import rs$line$2.{...}
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@619c93ca, import = import rs$line$2.{...}
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@619c93ca, import = import rs$line$1.{...}
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@3b2c4a8b, import = import rs$line$1.{...}
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@3b2c4a8b, import = import Predef.{...}
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@7f2c57fe, import = import Predef.{...}, implicits = ...
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@7f2c57fe, import = import scala.{...}
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@7f2c57fe, import = import java.lang.{...}
  owner = module class <root>, scope = dotty.tools.dotc.core.Scopes$MutableScope@7f2c57fe, import =
  owner = val <none>, scope = dotty.tools.dotc.core.Scopes$MutableScope@43935e9c, import =
  owner = val <none>, scope = null, import =
  owner = val <none>, scope = null, import =
  owner = val <none>, scope = null, import =
  owner = val <none>, scope = null, import =
  owner = val <none>, scope = null, import = )

Fixes #11146
Fixes #9720

[smarter]

Thanks for figuring that out!