Increase the package version to 5.4 (#205)

The latest Swift versions appear to have a problem with the
LinuxMain.swift again, which we haven't needed for quite some time.
Linux test autodiscovery has been enabled since 5.4, so we can
update to that version and drop the file.

Author: natecook1000

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.1
+// swift-tools-version:5.4
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the Swift Algorithms open source project