Remove unused minimum iOS version from cmake build.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: I70b88639db55f54bcdff1d87dadc8458a7c44b75

build/cmake/toolchains/arm-ios-common.cmake

diff --git a/build/cmake/toolchains/arm-ios-common.cmake b/build/cmake/toolchains/arm-ios-common.cmake
index 1e98928..8317ae2 100644
--- a/build/cmake/toolchains/arm-ios-common.cmake
+++ b/build/cmake/toolchains/arm-ios-common.cmake
@@ -18,11 +18,6 @@
 set(CMAKE_CXX_COMPILER clang++)
 set(CMAKE_CXX_COMPILER_ARG1 "-arch ${CMAKE_SYSTEM_PROCESSOR}")
 
-if (NOT AOM_MIN_IOS_VERSION)
-  set(AOM_MIN_IOS_VERSION 9.1)
-endif ()
-set(CMAKE_XCODE_ATTRIBUTE_IPHONEOS_DEPLOYMENT_TARGET ${AOM_MIN_IOS_VERSION})
-
 # Assembler sources must be converted for ARM iOS targets.
 set(AOM_ADS2GAS_REQUIRED 1)
 set(AOM_ADS2GAS "${CMAKE_CURRENT_SOURCE_DIR}/build/make/ads2gas_apple.pl")