removed sjasm sub module, added it in sgdk repo (will never change)

tools/sjasm/LICENSE.txt

@@ -0,0 +1,22 @@
+Sjasm Z80 Assembler
+
+Copyright (c) 2021 Konamiman
+Based on Sjasm 0.39g6 - Copyright (c) 2006 Sjoerd Mastijn
+
+This software is provided 'as-is', without any express or implied warranty.
+In no event will the authors be held liable for any damages arising from the
+use of this software.
+
+Permission is granted to anyone to use this software for any purpose,
+including commercial applications, and to alter it and redistribute it freely,
+subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not claim
+   that you wrote the original software. If you use this software in a product,
+   an acknowledgment in the product documentation would be appreciated but is
+   not required.
+
+2. Altered source versions must be plainly marked as such, and must not be
+   misrepresented as being the original software.
+
+3. This notice may not be removed or altered from any source distribution.