Update RxDataSources to build against Swift 4.2

[clackary]

Pull Request Description

Updates the RxDataSources configuration to use SwiftPM. The Xcode project configurations were removed because of a missing RxSwift dependency.

• pass ./project_precommit_check script run

$ ./project_precommit_check RxDataSources --earliest-compatible-swift-version 4.2
** CHECK **
--- Validating RxDataSources Swift version 4.2 compatibility ---
--- Project configured to be compatible with Swift 4.2 ---
--- Checking RxDataSources platform compatibility with Darwin ---
--- Platform compatibility check succeeded ---
--- Locating swiftc executable ---
$ xcrun -f swiftc
--- Checking installed Swift version ---
$ /Applications/Xcodes/PublicGMs/Xcode-10-GM.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc --version
--- Version check succeeded ---
--- Executing build actions ---
$ ./runner.py --swift-branch swift-4.2-branch --swiftc /Applications/Xcodes/PublicGMs/Xcode-10-GM.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc --projects projects.json --include-repos 'path == "RxDataSources"' --include-versions 'version == "4.2"' --include-actions 'action.startswith("Build")'
PASS: RxDataSources, 4.2, 545873, Swift Package
========================================
Action Summary:
     Passed: 1
     Failed: 0
    XFailed: 0
    UPassed: 0
      Total: 1
========================================
Repository Summary:
      Total: 1
========================================
Result: PASS
========================================
--- RxDataSources checked successfully against Swift 4.2 ---

[clackary]

@swift-ci test