Revert "[tls] Add boringssl, mbedtls and picotls library"

This reverts commit d99136c122f3b3e875fd6bfca82f1fe78e09bd20.

Reason for revert: Should use pigweed hosted boringssl/mbedtls libraries.

Original change's description:
> [tls] Add boringssl, mbedtls and picotls library
>
> Add boringssl, mbedtls and picotls libraries as submodules into
> third_party folder. Build scripts will be added later one by one.
>
> Change-Id: I15f100f53aac2c3980e7f12e1e9b6f8ddf008f10
> Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/37340
> Reviewed-by: Ali Zhang <alizhang@google.com>
> Commit-Queue: Yecheng Zhao <zyecheng@google.com>

TBR=vars@google.com,pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com,alizhang@google.com,zyecheng@google.com

Change-Id: I427de17facdd95caa606e2552ab19835d34155c5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/37289
Reviewed-by: Varun Sharma <vars@google.com>
Commit-Queue: Yecheng Zhao <zyecheng@google.com>
diff --git a/.gitmodules b/.gitmodules
index 42105ec..317cd91 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -4,15 +4,3 @@
 [submodule "third_party/nanopb"]
 	path = third_party/nanopb
 	url = https://pigweed.googlesource.com/third_party/github/nanopb/nanopb
-[submodule "third_party/boringssl/src"]
-	path = third_party/boringssl/src
-	url = https://boringssl.googlesource.com/boringssl
-	branch = master
-[submodule "third_party/mbedtls/src"]
-	path = third_party/mbedtls/src
-	url = https://github.com/ARMmbed/mbedtls.git
-	branch = development
-[submodule "third_party/picotls/src"]
-	path = third_party/picotls/src
-	url = https://github.com/h2o/picotls.git
-	branch = master
diff --git a/third_party/boringssl/src b/third_party/boringssl/src
deleted file mode 160000
index 04b3213..0000000
--- a/third_party/boringssl/src
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 04b3213d43492b6c9e0434d8e2a4530a9938f958
diff --git a/third_party/mbedtls/src b/third_party/mbedtls/src
deleted file mode 160000
index 17605f0..0000000
--- a/third_party/mbedtls/src
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 17605f072b6bc85e1a9cf6a81bd6b5d5eb65440a
diff --git a/third_party/picotls/src b/third_party/picotls/src
deleted file mode 160000
index 47327f8..0000000
--- a/third_party/picotls/src
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 47327f8d032f6bc2093a15c32e666ab6384ecca2