Remove gflags.

With the move to abseil's flags, this removed the now-unnecessary submodule and installation commands.

.gitmodules

@@ -8,9 +8,6 @@
 [submodule "third_party/protobuf"]
 	path = third_party/protobuf
 	url = https://github.com/google/protobuf.git
-[submodule "third_party/gflags"]
-	path = third_party/gflags
-	url = https://github.com/gflags/gflags.git
 [submodule "third_party/googletest"]
 	path = third_party/googletest
 	url = https://github.com/google/googletest.git

.travis.yml

@@ -47,7 +47,6 @@ before_install:
   - pod --version
   # Recent pods aren't found if we don't explicitly update Cocoapods' repo.
   - pod repo update
-  - brew install gflags
 install:
   - pushd $TEST_PATH
   - pod install

BUILDING.md

@@ -19,8 +19,6 @@ If you plan to build using CMake
 
 If you are a contributor and plan to build and run tests, install the following as well:
 ```sh
- $ # libgflags-dev is only required if building with make (deprecated)
- $ [sudo] apt-get install libgflags-dev
  $ # clang and LLVM C++ lib is only required for sanitizer builds
  $ [sudo] apt-get install clang-5.0 libc++-dev
 ```
@@ -45,12 +43,6 @@ packages from [Homebrew](https://brew.sh):
 
 If you plan to build using CMake, follow the instructions from https://cmake.org/download/
 
-If you are a contributor and plan to build and run tests, install the following as well:
-```sh
- $ # gflags is only required if building with make (deprecated) 
- $ brew install gflags
-```
-
 *Tip*: when building,
 you *may* want to explicitly set the `LIBTOOL` and `LIBTOOLIZE`
 environment variables when running `make` to ensure the version