Remove C library and sys crate (#139)

After merging d914ea4 and
c991797 the `swiftnav-sys` crate is no
longer used nor compiled. This PR finally removes the C library
submodule and crate from the repository.

Author: jbangelo

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "third-party/libswiftnav"]
-	path = swiftnav-sys/src/libswiftnav
-	url = https://github.com/swift-nav/libswiftnav.git

README.md

@@ -8,29 +8,9 @@ intended to be as portable as possible and has limited dependencies.
 Navigation receivers.  See [libsbp](https://github.com/swift-nav/libsbp) to
 communicate with receivers using Swift Binary Protocol (SBP).
 
-# swiftnav-sys
-
-`swiftnav-sys` is a crate which builds and exposes Rust FFI bindings for the
-`libswiftnav` C library.
-
 # Publishing a new release
 
-Releases are done against the master branch.  Use the `cargo publish` tool.  First
-release the `swiftnav-sys` crate:
-
-Update the `swiftnav-sys` crate version:
-
-```
-cd swiftnav-sys
-sed -i 's@version = "0.8.3"@version = "0.8.4"@' Cargo.toml
-```
-
-Then release the package:
-
-```
-cd swiftnav-sys
-cargo publish
-```
+Releases are done against the master branch.  Use the `cargo publish` tool.
 
 Update the `swiftnav` crate version: