Revert "Build with v2 extensions" / GOAMD64=v2

As seen in #908,
there are users still running x86-64-v1 cpus. They get

	This program can only be run on AMD64 processors
	with v2 microarchitecture support.

and cannot read their encrypted files.

Build for AMD64v1 again an accept the XChaCha20 performance hit for
for some cpus ( golang/go#67240 ).

This reverts commit f5007b2.

Related:
#828
#833
apptainer/apptainer#2873
golang/go#67240

Author: rfjakob

build.bash

@@ -85,9 +85,6 @@ if [[ -n ${LDFLAGS:-} ]] ; then
 	GO_LDFLAGS="$GO_LDFLAGS \"-extldflags=$LDFLAGS\""
 fi
 
-# Set GOAMD64 version to v2
-export GOAMD64=v2
-
 # Actual "go build" call for gocryptfs
 go build "-ldflags=$GO_LDFLAGS" "$@"
 # Additional binaries