Remove -cpu flag (#2793)

The -cpu flag is a weird one (and copied originally from Caddy), it
basically sets GOMAXPROCS which can be *easily* done by just setting
that environment variable. Also with systemd and containerized env you
set this externally *anyway*, so there is little use to do this again in
the binary.

Also the option's help was confusing (i.e. percentage of what?). Remove
the option and supporting files.

Signed-off-by: Miek Gieben <miek@miek.nl>

coremain/run_test.go

@@ -1,44 +0,0 @@
-package coremain
-
-import (
-	"runtime"
-	"testing"
-)
-
-func TestSetCPU(t *testing.T) {
-	currentCPU := runtime.GOMAXPROCS(-1)
-	maxCPU := runtime.NumCPU()
-	halfCPU := int(0.5 * float32(maxCPU))
-	if halfCPU < 1 {
-		halfCPU = 1
-	}
-	for i, test := range []struct {
-		input     string
-		output    int
-		shouldErr bool
-	}{
-		{"1", 1, false},
-		{"-1", currentCPU, true},
-		{"0", currentCPU, true},
-		{"100%", maxCPU, false},
-		{"50%", halfCPU, false},
-		{"110%", currentCPU, true},
-		{"-10%", currentCPU, true},
-		{"invalid input", currentCPU, true},
-		{"invalid input%", currentCPU, true},
-		{"9999", maxCPU, false}, // over available CPU
-	} {
-		err := setCPU(test.input)
-		if test.shouldErr && err == nil {
-			t.Errorf("Test %d: Expected error, but there wasn't any", i)
-		}
-		if !test.shouldErr && err != nil {
-			t.Errorf("Test %d: Expected no error, but there was one: %v", i, err)
-		}
-		if actual, expected := runtime.GOMAXPROCS(-1), test.output; actual != expected {
-			t.Errorf("Test %d: GOMAXPROCS was %d but expected %d", i, actual, expected)
-		}
-		// teardown
-		runtime.GOMAXPROCS(currentCPU)
-	}
-}