MNT: Drop gh-pages history, reducing repository size

[effigies]

Currently:

$ time git clone https://github.com/nipreps/niworkflows.git           
Cloning into 'niworkflows'...
remote: Enumerating objects: 17457, done.
remote: Counting objects: 100% (17457/17457), done.
remote: Compressing objects: 100% (2299/2299), done.
remote: Total 890088 (delta 15964), reused 16084 (delta 14654), pack-reused 872631
Receiving objects: 100% (890088/890088), 1.14 GiB | 8.75 MiB/s, done.
Resolving deltas: 100% (801905/801905), done.
git clone https://github.com/nipreps/niworkflows.git  225.60s user 34.76s system 128% cpu 3:22.09 total
$ du -sh niworkflows 
1.3G	niworkflows

Just checking out master:

$ time git clone --single-branch https://github.com/nipreps/niworkflows.git
Cloning into 'niworkflows'...
remote: Enumerating objects: 70, done.
remote: Counting objects: 100% (70/70), done.
remote: Compressing objects: 100% (59/59), done.
remote: Total 11181 (delta 12), reused 63 (delta 11), pack-reused 11111
Receiving objects: 100% (11181/11181), 286.47 MiB | 9.25 MiB/s, done.
Resolving deltas: 100% (7775/7775), done.
git clone --single-branch https://github.com/nipreps/niworkflows.git  15.85s user 8.86s system 76% cpu 32.486 total
$ du -sh niworkflows
386M	niworkflows

Takes advantage of tschaub/gh-pages#277. Using 3.0.0 to avoid tschaub/gh-pages#354, affecting 3.1.0.

[mgxd]

LGTM

[effigies]

Follow-up:

$ time git clone https://github.com/nipreps/niworkflows.git                
Cloning into 'niworkflows'...
remote: Enumerating objects: 8885, done.
remote: Counting objects: 100% (8885/8885), done.
remote: Compressing objects: 100% (1829/1829), done.
remote: Total 20314 (delta 8030), reused 7558 (delta 6812), pack-reused 11429
Receiving objects: 100% (20314/20314), 304.80 MiB | 5.22 MiB/s, done.
Resolving deltas: 100% (15916/15916), done.
git clone https://github.com/nipreps/niworkflows.git  20.76s user 11.33s system 52% cpu 1:01.04 total
$ du -sh niworkflows 
404M	niworkflows