build: Remove custom file overrides for hash/crc32 package.

By now, the crc32 package makes use of functions implemented
in architecture-specific files. GopherJS ends up taking on the
generic "other architectures" configuration that works when
there isn't an architecture-specific CRC32-IEEE algorithm,
nor an architecture-specific CRC32-C algorithm.

In the future, it's possible hash/crc32 performance can be improved
by doing something architecture-specific, but this will do for now.

Fixes the following build issue for hash/crc32 package:

	$ gopherjs build hash/crc32
	../../../usr/local/go/src/hash/crc32/crc32.go:83:23: undeclared name: archAvailableCastagnoli
	../../../usr/local/go/src/hash/crc32/crc32.go:86:3: undeclared name: archInitCastagnoli
	../../../usr/local/go/src/hash/crc32/crc32.go:87:22: undeclared name: archUpdateCastagnoli
	../../../usr/local/go/src/hash/crc32/crc32.go:107:17: undeclared name: archAvailableIEEE
	../../../usr/local/go/src/hash/crc32/crc32.go:110:3: undeclared name: archInitIEEE
	../../../usr/local/go/src/hash/crc32/crc32.go:111:16: undeclared name: archUpdateIEEE

Author: dmitshur

build/build.go

@@ -95,8 +95,6 @@ func importWithSrcDir(path string, srcDir string, mode build.ImportMode, install
 		pkg.GoFiles = []string{"rand.go", "util.go"}
 	case "crypto/x509":
 		pkg.CgoFiles = nil
-	case "hash/crc32":
-		pkg.GoFiles = []string{"crc32.go", "crc32_generic.go"}
 	}
 
 	if len(pkg.CgoFiles) > 0 {