Remove unused -XCPP

[philderbeast]

Fixes #10720, a follow on from #10092. Removes unused {-# LANGUAGE CPP #-} pragmas.

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Template B: This PR does not modify behaviour or interface

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

Manual QA Notes

Please delete dist-newstyle and run the following to check for occurrences of unused -XCPP:

$ grep '^#if' --files-without-match $(grep -R --files-with-matches 'LANGUAGE.*CPP' */**/*.hs)
cabal-testsuite/PackageTests/Backpack/bkpcabal01/p/P.hs
cabal-testsuite/PackageTests/CabalMacros/Mdl.hs
cabal-testsuite/PackageTests/CmmSources/src/Demo.hs
cabal-testsuite/PackageTests/CmmSourcesDyn/demo/Main.hs
cabal-testsuite/PackageTests/CmmSourcesDyn/src/Demo.hs
cabal-testsuite/PackageTests/CmmSourcesExe/src/Demo.hs
cabal-testsuite/PackageTests/Haddock/CPP.hs
cabal-testsuite/PackageTests/PreProcess/Hsc2HsOptionsCC/Main.hs
cabal-testsuite/PackageTests/Regression/T5386/Foo.hs
Cabal/src/Distribution/Simple/Build/PathsModule/Z.hs

$ grep 'LANGUAGE CPP' Cabal/src/Distribution/Simple/Build/PathsModule/Z.hs
    tell "{-# LANGUAGE CPP #-}\n"

Those in cabal-testsuite/PackageTests are part of tests so should likely stay put and Distribution.Simple.Build.PathsModule.Z is generating a paths module using a zinza template. It's a false positive find with the unrefined grep pattern, LANGUAGE.*CPP.