arm: Get rid of using namespace std and TheISA in realview.cc.
arm: Get rid of using namespace std and TheISA in realview.cc.

Neither was actually used by the nearly empty file.

Change-Id: Ief1b77b18c8c616511bf9870e1667439a42dfcbd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22266
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Maintainer: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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