Remove 3rdparty submodule and internal_only build option

Remove 3rdparty submodule and allow to build external tests, examples
and benchmarks using the Compute Library test framework.

In order to use this option, the external tests directory must have the
following structure:

EXTERNAL_TESTS_DIR:
└── tests
    ├── benchmark
    │   ├── CL
    │   ├── datasets
    │   ├── fixtures
    │   └── NEON
    └── validation
        ├── CL
        ├── datasets
        ├── fixtures
        └── NEON

Then, the user can build the library with
`external_tests_dir=<PATH_TO_EXTERNAL_TESTS_DIR>`.

Also, remove internal_only build option". Without the 3rdparty
submodule, this build option is no longer needed.

Resolves: COMPMID-3417, COMPMID-3419

Change-Id: Ib7bf8ec7b9b9317fd82012a96bf4a7b0be846c1b
Signed-off-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4704
Reviewed-by: Georgios Pinitas <georgios.pinitas@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
diff --git a/SConstruct b/SConstruct
index 15197a6..b543bc0 100644
--- a/SConstruct
+++ b/SConstruct
@@ -64,8 +64,7 @@
     PathVariable("install_dir", "Specify sub-folder for the install", "", PathVariable.PathAccept),
     BoolVariable("exceptions", "Enable/disable C++ exception support", True),
     PathVariable("linker_script", "Use an external linker script", "", PathVariable.PathAccept),
-    #FIXME Remove before release (And remove all references to INTERNAL_ONLY)
-    BoolVariable("internal_only", "Enable ARM internal only tests", False),
+    PathVariable("external_tests_dir", "Add examples, benchmarks and tests to the tests suite", "", PathVariable.PathAccept),
     ListVariable("custom_options", "Custom options that can be used to turn on/off features", "none", ["disable_mmla_fp"]),
     ListVariable("data_type_support", "Enable a list of data types to support", "all", ["qasymm8", "qasymm8_signed", "qsymm16", "fp16", "fp32"]),
     ("toolchain_prefix", "Override the toolchain prefix", ""),