Makefile: remove separate linux build targets for different architecture

[anjandev]

This makes packaging on linux much harder. Go build already detects the correct architecture. Using this patch, you can create and cross compile binaries for linux as such:

pinephone:~/code/comics-downloader$ export GOARCH=arm64 pinephone:~/code/comics-downloader$ rm -rf build/ pinephone:~/code/comics-downloader$ make linux-build pinephone:~/code/comics-downloader$ export GOARCH=amd64 pinephone:~/code/comics-downloader$ rm -rf build/ pinephone:~/code/comics-downloader$ make linux-build

closes #103

[coveralls]

Pull Request Test Coverage Report for Build 120

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 57.632%

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Totals
Change from base Build 119:	0.0%
Covered Lines:	740
Relevant Lines:	1284

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💛 - Coveralls