travis: revove rtos support for NUCLEO_L053R8

Author: jeromecoutant

tools/build_travis.py

@@ -36,7 +36,7 @@
     { "target": "LPC11U24_301",      "toolchains": "GCC_ARM", "libs": ["fat"] },
 
     { "target": "B96B_F446VE",     "toolchains": "GCC_ARM", "libs": ["dsp", "fat"] },
-    { "target": "NUCLEO_L053R8",     "toolchains": "GCC_ARM", "libs": ["dsp", "rtos", "fat"] },
+    { "target": "NUCLEO_L053R8",     "toolchains": "GCC_ARM", "libs": ["dsp", "fat"] },
     { "target": "NUCLEO_L152RE",     "toolchains": "GCC_ARM", "libs": ["dsp", "rtos", "fat"] },
     { "target": "NUCLEO_F030R8",     "toolchains": "GCC_ARM", "libs": ["dsp", "fat"] },
     { "target": "NUCLEO_F031K6",     "toolchains": "GCC_ARM", "libs": ["dsp", "fat"] },