dev-libs/mtxclient: Remove old ebuild and patch.

Signed-off-by: Ronny (tastytea) Gutbrod <gentoo@tastytea.de>

dev-libs/mtxclient/files/0.3.0_add_missing_header.patch

@@ -1,22 +0,0 @@
-From 3692a1dce868cfde29547757b6a8e91a05761b12 Mon Sep 17 00:00:00 2001
-From: Nicolas Werner <nicolas.werner@hotmail.de>
-Date: Sat, 9 May 2020 00:33:29 +0200
-Subject: [PATCH] gcc 10 fixes
-
----
- include/mtx/identifiers.hpp | 2 ++
- tests/messages.cpp          | 2 +-
- 2 files changed, 3 insertions(+), 1 deletion(-)
-
-diff --git a/include/mtx/identifiers.hpp b/include/mtx/identifiers.hpp
-index a18b26e9..077099c4 100644
---- a/include/mtx/identifiers.hpp
-+++ b/include/mtx/identifiers.hpp
-@@ -6,4 +6,6 @@
- #include <nlohmann/json.hpp>
- #endif
-
-+#include <stdexcept>
-+
- namespace mtx {
- namespace identifiers {