Drop workarounds for Go 1.11 and Go 1.12

Makefile

@@ -51,7 +51,7 @@ ci:
 	go clean -modcache
 	# GOPROXY=off makes sure we fail instead of making network requests
 	# (we should not need any!)
-	# "-mod=vendor" is required for Go 1.11
+	# "-mod=vendor" is required for Go 1.13
 	GOPROXY=off ./build.bash -mod=vendor
 	# Delete "vendor" dir
 	rm -R vendor

build.bash

@@ -15,10 +15,6 @@ cd "$(dirname "$0")"
 # Make sure we have the go binary
 go version > /dev/null
 
-# Enable Go Modules on Go 1.11 and 1.12
-# https://dev.to/maelvls/why-is-go111module-everywhere-and-everything-about-go-modules-24k#-raw-go111module-endraw-with-go-111-and-112
-export GO111MODULE=on
-
 # GOPATH may contain multiple paths separated by ":"
 GOPATH1=$(go env GOPATH | cut -f1 -d:)
 
@@ -65,15 +61,7 @@ fi
 
 # Only set GOFLAGS if it is not already set by the user
 if [[ -z ${GOFLAGS:-} ]] ; then
-	GOFLAGS=""
-	# For reproducible builds, we get rid of $HOME references in the
-	# binary using "-trimpath".
-	# However, -trimpath needs Go 1.13+, and we support Go 1.11 and Go 1.12
-	# too. So don't add it there.
-	GV=$(go version)
-	if [[ $GV != *"1.11"* && $GV != *"1.12"* ]] ; then
-		GOFLAGS="-trimpath"
-	fi
+	GOFLAGS="-trimpath"
 	# Also, Fedora and Arch want pie enabled, so enable it.
 	# * https://fedoraproject.org/wiki/Changes/golang-buildmode-pie
 	# * https://github.com/rfjakob/gocryptfs/pull/460