Revert "Enable the parallel feature of cc"

This reverts commit 92491db.

Disable parallel feature of cc by default which breaks build of vendored
openssl.

Fixes #1026

Author: IGI-111

libgit2-sys/Cargo.toml

@@ -27,7 +27,7 @@ libz-sys = { version = "1.1.0", default-features = false, features = ["libc"] }
 
 [build-dependencies]
 pkg-config = "0.3.15"
-cc = { version = "1.0.43", features = ['parallel'] }
+cc = "1.0.43" # parallel feature is not compatible with vendored openssl
 
 [target.'cfg(unix)'.dependencies]
 openssl-sys = { version = "0.9.45", optional = true }