archive: remove pgzip, use gzip from go std lib (MR 25)

This replaces the parallel gzip with the boring gzip from Go's standard
lib. The main motivations for doing this are:

1) Reduce runtime memory requirements

2) shed some external dependencies

There's obviously a trade-off with compression speed/time (as seen
below), but I feel like it's a worthwhile trade-off.

Note that there's likely very little impact to boot performance wrt
extracting these archives, the compression levels are similar.

Measured on a Shift 6mq, which is a very fast phone...

** compress/gzip:

User time (seconds): 1.81
System time (seconds): 0.38
Percent of CPU this job got: 104%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:02.09
Maximum resident set size (kbytes): 62024

-rw-r--r-- 1 clayton clayton 6.1M Sep 20 17:20 initramfs
-rw-r--r-- 1 clayton clayton 2.5M Sep 20 17:20 initramfs-extra

** pgzip:

User time (seconds): 1.19
System time (seconds): 0.48
Percent of CPU this job got: 159%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:01.05
Maximum resident set size (kbytes): 109952

-rw-r--r-- 1 clayton clayton 6.8M Sep 20 17:20 initramfs
-rw-r--r-- 1 clayton clayton 2.8M Sep 20 17:20 initramfs-extra

inspired by: https://gitlab.com/postmarketOS/pmaports/-/issues/1704

go.mod

@@ -4,7 +4,5 @@ go 1.16
 
 require (
 	github.com/cavaliercoder/go-cpio v0.0.0-20180626203310-925f9528c45e
-	github.com/klauspost/compress v1.13.3 // indirect
-	github.com/klauspost/pgzip v1.2.5
 	golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c
 )